Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 241

mineral chemistry and relationships
237
The biotites of individual rock samples are more homogeneous in com-
position than are the phlogopites of individual rock samples. This holds for
both the Mg/Fe ratio and the analysed elemental oxides.
Si02 (Fig. 118) is slightly higher in the phlogopites than in the biotites. In
the most magnesian phlogopites the SÍO2 content is mostly around 39 wt.
per cent. In the less magnesian phlogopites and the more magnesian
biotites, on the other hand, a SiOz content of around 37 wt. per cent is more
common, decreasing towards a 35 per cent level in the most iron rich
biotites.
AI2O3 (Fig. 118) is slightly lower in the phlogopites than in the biotites. It
is mostly between 12 and 13 wt. per cent in the phlogopites but 14 and 15
per cent in the biotites. This is in agreement with the tetrahedral Si-Al
substitution mentioned above. There are two exceptions to this higher AI2O3
content in biotites than phlogopites. These are the biotites of the trachyte
sample, Jan 81, which are not primary phenocryst biotites, and the biotites
of the hydrous mineral xenolith, Jan 329.
Ti02 (Fig. 118) varies from 4 to nearly 10 wt. per cent in the micas of the
Jan Mayen rocks. In the case of the phlogopites the range is slightly
narrower and the average slightly higher than in the case of the biotites. The
phlogopites of individual rock samples reveal a clear trend ol increasing
l’i02 content with decreasing Mg/Fe ratio. Since the Mg/he ratio decreases
with continued crystallization and increasing difíerentiation of magmas, as
has been shown by Rimsaite (1967), this means that the phlogopite takes up
tnore Ti as the crystallization continues. In individual rock samples contain-
ing biotite, the biotites are more homogeneous in composition regarding
ri02 content as well as other components.
K20 (Fig. 118) content of the phlogopites is mostly between 10 and 12 wt.
per cent, while in the biotites and some of the less magnesian phlogopites,
K20 values between 8 and 10 per cent are most usual. I his high amount of
K in the phlogopites results in some K in excess of the 2.000 required by the
structural formula. This is most probably a consequence of the complex
substitution in the Mg-Fe2+ sites of the structure; the presence of Ii4+ in
great amounts results in some excess K+ to ease the charge balance. 1 he
amount of Ti in the biotites is quite similar to that of the phlogopites, but in
the biotites the amount of Si is less than in the phlogopites. This lower Si4+
decreases the balance problem caused by the Ti incorporation and no
uptake of excess K is necessary.
Na20 (Fig. 118) is rather constant, varying between 0.6 and 1.0 wt. per
cent in both phlogopites and biotites. There is, however, a tendency towards
a slight Na decrease with decreasing Mg/Fe ratio. This decrease is clear in
the calculated average values for the micas of individual samples.
CaO (Fig. 119) is extremely low in the micas, in all cases being less than
0.2 wt. per cent. On the average it is slightly higher in the phlogopites than
in the biotites. In 20 per cent of the analysed phlogopites and in 38 per cent